Reference light adjustment method for spectrometer based measurement or control systems

ABSTRACT

A method to adjust the energy transmitted from a multiplicity of light sources to provide an adequate reference for spectral measurement or control using a multichannel feedback adjustment algorithm that compensates for the interactions between adjacent spectral ranges and sets reference light sources for optimal system performance using a normalized energy value for each spectral range is disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of previously filed co-pending Provisional Patent Application, Ser. No. 62/525,799, filed on Jun. 28, 2017.

FIELD OF THE INVENTION

The method of this disclosure belongs to the field of sample measurements or control systems using spectrometers. More specifically it is a method using a multichannel feedback adjustment system to modify the energy transmitted from a multiplicity of light sources, such that an adequate reference value (in counts, which represent quantities of light energy) is provided for each pixel (or range of wavelengths) of a spectrometer being used as a component in a measurement or control system.

BACKGROUND OF THE INVENTION

The optical path for a typical spectrometer based measurement or control system exhibits change due to mechanical variances caused by temperature, vibration, shock or wear. Examples of these changes include: optical variations, such as fiber solarization or lens deposits among others; and electrical drift from temperature changes, component aging or design tolerances. Providing consistent reference values for all pixels across the spectrum of interest improves system stability, increases mean time between maintenance, raises robustness, and can reduce component cost. The most common method for establishing light source intensity is to set the integration time of the spectrometer such that the most intense wavelength results in a count value slightly less than the saturation count for that pixel. Of course the remaining pixels then suffer from lower signal to noise ratios. Thus, a method that ensures consistent and appropriate reference light source intensity for a spectrometer based measurement or control system is needed.

BRIEF SUMMARY OF THE INVENTION

This application discloses a multichannel feedback adjustment system that modifies the energy transmitted from a multiplicity of light sources to provide an adequate reference intensity for spectral measurement or control systems. A control algorithm compensates for the interactions between adjacent spectral ranges and sets the intensity of the reference light sources for optimal system performance.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The method of this disclosure provides a mechanism by which the reference light energy for a spectrometer based measurement or control system is adjusted appropriately for spectral ranges of interest, such that the counts reported by the spectrometer are consistent (and near saturation counts) during its usage duty cycle.

The preferred embodiment of the disclosed method achieves the aforementioned purpose by implementing a feedback loop formed by a spectrometer, a light delivery optical system, the system under test and an adjustable multispectral light source. At the beginning of each control or measurement period, the source light intensity is set for optimal system performance. During this reference step, the spectrometer measures light intensity across its entire spectral range. It integrates the normalized energy measured for each spectral channel represented by the multispectral light source. Adjacent spectral channel adjustments are calculated and intensity settings sent to the reference light source. Once again, the light is measured, adjacent spectral channel adjustments made and settings sent. When the values settle, the intensity counts for each spectral channel should be similar. Thus, rather than adjusting the spectrometer integration time to compensate for losses in the optical system, the reference light source spectral channel intensities are adjusted.

Through the use of this multichannel feedback method a spectrometer based measurement or control system can be kept at a high level of performance automatically, eliminating the need for users to understand the complexities of the optical system.

More specifically in the preferred embodiment the number of counts for each pixel can be thought of as representing a normalized energy. Given a target value for a spectral channel of interest, the corresponding light source intensity is then set to cancel losses in the optical path. Rather than independently ramping the value of each spectral channel to match losses, as is typical in the prior art, a multichannel feedback adjustment algorithm compensates for the effects of light energy from adjacent spectral channels. All spectral channels are adjusted simultaneously to account for interactions between them resulting in more repeatable settings. By using a feedback system, such as a multichannel Proportional-Integral-Derivative controller, well known by those skilled in the art, the effects of adjacent spectral channels are essentially eliminated when the loop reaches equilibrium reducing the need for complex iterative calculations.

Identified user issues which will be solved by the disclosed feedback method include: Spectrometer, fiber, and light source response is different from unit to unit; fibers degrade over time, at different wavelengths and at different rates; light sources degrade over time at different wavelengths and at different rates; and users need to understand all of this and make allowances for these variations at the system level.

By using the intensity counts from the spectrometer to adjust the input levels of the reference light source intensity at each spectral channel, the spectrometer measurement or control system will automatically be able to adjust for differences in the response of each spectrometer quickly and easily without technical knowledge. It will also adjust as fibers or light sources degrade, by increasing the energy in the specific wavelengths that have suffered decreased intensity. This frees a user from thinking about how the light source, fiber, and spectrometer efficiencies interplay and how they change over time, allowing for quick customization of a spectrum profile, while taking into account all three subsystems of a typical spectrometer measurement or control system. For example, this method comprehends different spectrum profiles for different measurements in the same system (i.e. only turn on the 350 nm LED if the measurement requires an absorption spectrum in that spectral range, and later the 500-600 nm LED for separate absorption spectrum so that there is no interaction).

Since certain changes may be made in the above described method for using a multichannel feedback adjustment algorithm for adjusting a reference light source without departing from the scope of the invention herein involved, it is intended that all matter contained in the description thereof shall be interpreted as illustrative and not in a limiting sense. 

What is claimed is:
 1. A method for adjusting an adjustable multiple spectral channel reference light source prior to starting a control or measurement period by implementing a feedback loop formed by a spectrometer, a light delivery optical system, a system under test, and an adjustable multiple spectral channel reference light source comprising: first, at the beginning of each control or measurement period, said adjustable multiple spectral channel reference light source's light intensity being set for optimal system performance and said spectrometer measuring light intensity across said spectrometer's entire spectral range; then said spectrometer measuring each spectral channel of said adjustable multiple spectral channel reference light source's quantities of normalized light energy and integrating said normalized light energy measured into intensity counts for each spectral channel contained in said adjustable multiple spectral channel reference light source; then calculating adjacent channel spectral intensity adjustments; then sending said adjacent channel spectral intensity adjustments to said adjustable multiple spectral channel reference light source; then adjusting each of said spectral channels of said adjustable multiple spectral channel reference light source; and, then repeating said channel measuring, said spectral intensity adjustment calculating, said sending of calculated adjustments, and said adjusting steps one or more times until said intensity counts for each spectral channel contained in said adjustable multiple spectral channel reference light source are similar. 